The Blinovitch Limitation Effect is a fictional principle of time travel physics in the universe of the long-running British science fiction television series Doctor Who. It is usually understood as having two aspects: firstly, that a time traveller cannot "redo" an act that they have previously committed, and secondly, that a dangerous energy discharge will result if two temporal versions of the same person come into contact. The effect was introduced in Day of the Daleks (1972), when Jo Grant asks the Third Doctor why a group of time-travelling guerrillas on a mission to assassinate a diplomat cannot simply go back into the past and try again if they fail. In reply, the Doctor cites the principle and begins to explain, but is interrupted before he can go any further. The effect was invented by script editor Terrance Dicks and producer Barry Letts to gloss over the plot problems inherent in the time travel premise of the serial. The interruption introduced by the writers meant that the explanation did not have to be expanded upon. The Effect is next mentioned in Invasion of the Dinosaurs (1974), where the Third Doctor states that it "tends to limit research into time travel" but, once again, he does not go into detail as to why. The novelisation of the story reveals that Blinovitch was a "great bear of a man from Russia" who had reversed his own timestream, reverting to infancy. The next time the effect is mentioned on-screen is in Mawdryn Undead (1983), but there it is not a scientific principle that prevents people from redoing their actions (for whatever reason). Rather, it is a physical effect that occurs when two versions of the same person from different time periods make physical contact. This results in an energy discharge, shorting out the "time differential" between them. The Mawdryn Undead storyline establishes that the younger version of the character involved in the discharge, Brigadier Lethbridge-Stewart, is traumatized by the event and, for the next several years, loses his memory of the Doctor. It is possible, however, as Lawrence Miles and Tat Wood theorise in their reference book About Time 5, that the energy discharge is simply a side effect of the effect's operation. It also appears, given the number of times that the Doctor has met his other incarnations, that the effect does not apply to Time Lords or, at the very least,